I have a 2001 Santa Fe 2.7 V6. While driving at cruise speed the (in 4th) the trans will suddenly drop into 3rd gear. This usually happens with a severe "thump", and then locks into 3rd gear. I have to pull over, turn the car off and wait for a bout 30 second. before I can restart. normally it it will be fine for a while, but every so often it will reoccur about 5 minutes later. and I will have shutdown again to "reset" the transmission.

I have taken the car to several trans specialists and to the local Hyundai dealer and they cannot find anything wrong with the transmission. The specialists have service the transmission (replaced belts, filters, strainers, oils, etc, and reset the computor) and the problem still occurs. The Hyundai dealers have run a full diagnostic on the car and found nothing, They say they have never heard of this occuring before.

Over the last month, the car has been to the service center about 8 times with the same problem....

Does anyone know what the cause could be? and how I might get it sorted?

 

The tranny is locked into 3rd, because the TCM has recognised a sensor fault. It generally is caused by a signal glitch of the output speed sensor (PG-B). This could be just a poor contact at the sensor, or the sensor tip is contaminated with some metalic dust (which is normally ocuring, since this sensor point at the output gear). There is generally a code in the TCM, but not always, specially when the problem hapen intermittently.
Have the tech remove and check the speed sensors, PG-A & B.